1. Field of the Invention
This invention relates generally to formation fluid sampling, and more specifically to a chamber in a downhole tool for collecting and storing a sample of formation fluid.
2. Description of the Related Art
The desirability of taking downhole formation fluid samples for chemical and physical analysis has long been recognized by oil companies, and such sampling has been performed by the assignee of the present invention, Schlumberger, for many years. Samples of formation fluid, also known as reservoir fluid, are typically collected as early as possible in the life of a reservoir for analysis at the surface and, more particularly, in specialized laboratories. The information that such analysis provides is vital in the planning and development of hydrocarbon reservoirs, as well as in the assessment of a reservoir's capacity and performance.
The process of wellbore sampling involves the lowering of a sampling tool, such as the MDT™ formation testing tool, owned and provided by Schlumberger, into the wellbore to collect a sample or multiple samples of formation fluid by engagement between a probe member of the sampling tool and the wall of the wellbore. The sampling tool creates a pressure differential across such engagement to induce formation fluid flow into one or more sample chambers within the sampling tool. This and similar processes are described in U.S. Pat. Nos. 4,860,581; 4,936,139 (both assigned to Schlumberger); U.S. Pat. Nos. 5,303,775; 5,377,755 (both assigned to Western Atlas); and U.S. Pat. No. 5,934,374 (assigned to Halliburton).
The desirability of housing at least one, and often a plurality, of such sample chambers, with associated valving and flow line connections, within “sample modules” is also known, and has been utilized to particular advantage in Schlumberger's MDT tool. Schlumberger currently has several types of such sample modules and sample chambers, each of which provide certain advantages for certain conditions.
There is strong desire in the formation sampling market for cleaner samples that are taken under controlled conditions that are held as close as possible to true formation conditions, and for the sample to be maintained at these conditions until withdrawn from the wellbore and then transported to a laboratory for analysis. Current sampling techniques use either a pump or formation pressure to drive the formation fluid sample into a vessel such as a sample chamber, displacing a piston in the vessel as the formation fluid flows in. The piston in the vessel is passive and is moved by the fluid. In some designs, after the sample is taken and confined, pressure is applied to the other side of the piston by a gas charging system or by the borehole hydrostatic pressure to compress the sample in order to increase or maintain the sample at a given pressure for transport. Such attempts have produced only limited success.
To address this shortcoming, it is a principal object of the present invention to provide an apparatus and method for bringing a high quality formation fluid sample to the surface for analysis. It is a further object of the present invention to provide techniques for controlling the pressure of a collected formation fluid sample within the sample chamber. It is desirable that such a system eliminate the need for additional valves, additional power requirements and/or additional cost. To this end, another object of the present invention is to provide a configuration capable of functioning with only one flowline valve to lock in a sample, and that an actuator be provided that is capable of operating a piston and the required valve(s). It is also desirable to have a system that is capable of gathering fluids from and/or injecting fluids into the formation.